1. Field of the Invention
This invention relates to the recovery of substantially pure triethylenediamine; and more particularly to a process for recovering a triethylenediamine solute in a liquid tertiary amine solution directly from a crude triethylenediamine reaction product mixture.
2. Description of the Prior Art
Triethylenediamine (TEDA) is a valuable commercial product, particularly as an accelerator or catalyst in conventional urethane systems employing a wide variety of isocyanates and polyols as reactants. Several methods for preparing triethylenediamine are well known. For example, one process is described by O. Hromatka et al. in Berichter Volume 76, pages 712-717 (1943) wherein triethylenediamine is obtained by the process of heating the dihydrochloride of N-(2-hydroxyethyl)piperazine. Another process involves the gaseous phase cyclization of N-hydroxyethylpiperazine vapor in the presence of a solid acid catalyst. Another well-known process is described in U.S. Pat. No. 3,080,371 to Spielberger et al which includes the liquid phase process of heating N-(2-hydroxyethyl)piperazine in the presence of a mono- or dicarboxylic acid catalyst at a temperature of from about 230.degree. to about 350.degree. C.
Generally, such well-known processes result in the formation of crude reaction product mixtures containing the triethylenediamine, water, by-product such as piperazine and high molecular weight polymers, catalyst and solvents, if any are employed. Triethylenediamine is normally distilled from the crude reaction product by fractional distillation followed by one or more crystallization steps. The substantially pure solid triethylenediamine thus recovered is then dissolved in a suitable solvent for use as a urethane catalyst.
These generally described conventional techniques for recovering triethylenediamine have several disadvantages. Pure triethylenediamine has a freezing point of 159.8.degree. C. and a boiling point of 174.degree. C. Pure triethylenediamine thus is normally a liquid over a very narrow temperature range of 14.2.degree. C. In view of this fact, it is extremely difficult to separate triethylenediamine from its crude reaction product mixtures by conventional techniques other than by crystallization. For example, pure triethylenediamine cannot be readily separated from reaction mixtures by conventional distillation techniques. Further, triethylenediamine readily freezes in the distillation equipment including condensation apparatus, vent lines, and the like, causing an equipment blockage problem. Solid, e.g., crystallized triethylenediamine is also somewhat difficult to work with. For example, the crystalline material tends to hydrate. Further, the solid compound has a slight odor requiring the use of special handling equipment in some cases.
In as much as conventional urethane systems normally utilize liquid reaction components and the solid triethylenediamine is difficult to handle, store and ship, the solid is normally dissolved in a suitable solvent such as a glycol which is compatible with urethane systems. Such triethylenediamine solutions have been prepared in a number of ways. For example, a substantially pure solid triethylenediamine may be obtained by the methods previously described herein and then dissolved in a suitable solvent. Such technique has the disadvantages as just described, particularly involving the need for crystallization and for handling a solid triethylenediamine. In yet other methods, such as that described in U.S. Pat. No. 3,993,651, the solvent employed has also been used as part of a purification scheme. However, such solvent, of course, is not an active catalyst in preparing a subsequent polyurethane. Thus, such solvent must necessarily be shipped along with the active triethylenediamine catalyst ingredient which involves additional costs.